It is important to remember that yin and yang are not static concepts and that they are constantly influencing and determining one another. There is always some measure of yin within yang and vice versa. To use the analogy of a hillside; during the day the sunlit side of the hill is yang within yang, while the shaded side is yin within yang. Conversely, at night the moonlit side of the hill is yang within yin while the dark side of the hill is yin within yin. In this fluid model it must be understood that neither yin nor yang can ever exist without the other. In fact, extreme yin will engender yang, an example of this can been seen in the popular expression "the darkest hour is right before the dawn". Naturally, the opposite is also true.
These types of relationships become significant when they impact the body's anatomy and physiology and it is precisely these designations that are used in the diagnosis of imbalances in TCM. For a TCM practitioner, the name of the disease is of secondary importance. The primary key to the proper diagnosis of syndromes is the identification of the condition in terms of yin or yang. In order to understand what this means let us examine these concepts in the context of human life.
Beginning at conception the sperm, which is yang, unites with the yin ovum and a new life is formed. As that life develops and progresses the energetic stages of youth are yang; whereas the later years are yin as life slows and becomes more deliberate. Each stage is also relative to the others and contains a measure of both yin and yang, just as the aforementioned hillside is an expression of yin within yang, etc. For example, the quick growth of early childhood is yang within yang and the transition from middle age to old age is yin within yang.
We can also see this philosophy expressed in everyday life. In respiration, the expansion of inhalation is yang while the emptiness which results from exhalation is yin. In digestion, the yin substance of food is transformed by the metabolic activity of yang. It is then converted into Qi (yang) and Blood (yin). Qi and Blood interact with one another using this paradigm. Qi moves Blood, yet Blood is thought to be the "mother" or source of Qi. Within the body yin is expressed as the material basis, the tissue and substance without which the transformation of yang would not be possible.
The physical body itself expresses this model. The lower part of the body which connects to the earth is yin while the upper body and extremities are yang and free to move. The front, which can easily be protected, is yin while the exposed back is yang. The internal organs, which are enclosed and protected, are yin relative to the surface of skin and muscle which are yang. In addition, the internal organs can be further differentiated into fu (yang), which are the "hollow" organs that are involved with digestion and elimination, and zang (yin) which are involved in assimilation and storage. Each zang has a corresponding fu organ which it is paired with and while these connections are not recognized in Western medical terms, they are often utilized in the treatment of disease in TCM. 医 学 全 在线www.med126.com
Finally, disease and disease progression can be viewed using this paradigm. If the body's yang is weak it will be unable to ward off the invasion of a pathogen. If the yin is weak there will not be enough nourishment and support for the yang and the result will be the same. Expressed in other terms, without the substance, the active immune system is weakened and without activity the substance becomes vulnerable. Therefore, if yin is deficient over time then yang also becomes deficient and vice versa. Not only do yin and yang balance each other, they mutually generate one another. It is precisely this balance that the TCM practitioner uses various treatment strategies to restore. The idea is to reestablish the body's innate ability to maintain health and defend itself from disease.
